High temperature thermal processes, for example, the generation of steam for the production of electricity in power plants utilizing fossil fuels, often create environmentally harmful by-products. These compounds, specifically nitrous oxides (NOx), have to be removed from the flue gases of the high temperature thermal process before being discharged to the environment. The standard for removing NOx from flue gases is the selective catalytic reduction (SCR) process, where a reducing reagent, typically ammonia, is injected, mixed into the flue gas, and sent through a catalytic reaction chamber where the catalyst facilitates the reduction of NOx with the reducing agent to form elemental nitrogen and water. The catalyst, often referred to as a DeNOx catalyst, aids these reactions and is typically constructed of titanium dioxide containing the oxides of transition metals such as, for example, vanadium, molybdenum, and tungsten to act as catalytically active components. The catalyst is arranged on plates, in a honeycomb fashion or as a corrugated structure and placed parallel to the direction of flue gas flow. However, during operation, the catalyst undergoes a loss of activity and efficiency as a result of plugging with ash and deactivation of the active components from certain compounds contained in flue gas which are poisonous to the catalyst. Among these are, for example, sodium (Na), potassium (K), arsenic (As), phosphorous (P) and many others.
It is generally known in the regeneration of SCR catalysts, that inorganic acids may be used to clean and restore a catalyst such as in a soaking step and a neutralization step. Typically, inorganic acids such as sulfuric acid (H2SO4) and hydrochloric acid (HCl) are commonly used inorganic acids in such steps. Sulfuric acid is relatively inexpensive and readily available. Inorganic acids are typically odorless which is another reason why they are commonly used. H2SO4 is also used due to the fact that SO2 and SO3 is present in the flue gas and collected by the catalyst during operation in a power plant facility and when a catalyst is submerged in an aqueous solution such as water, it is removed from the catalyst forming a diluted sulfuric acid. Thus, it is already present in the catalyst and new contaminants via other acids are not being introduced into the catalyst. However, the known methods are also disadvantageous because diluted H2SO4 also corrodes the steel casings of the catalyst. This provides water soluble iron substances which can penetrate in the inside of the pores of a SCR-catalyst. These iron ions are probably the main cause of enhancing some of the undesired side reactions during regular SCR operations in a power plant. During the most undesired of these side reactions, the contents of SO2 in the flue gas can be partially oxidized to SO3, which reacts with water (H2O) in the flue gas to produce H2SO4. Corrosion of all steel surfaces at temperatures below the H2SO4 dew point is a problem for all equipment downstream of the catalyst. Furthermore, the possible emission of visible H2SO4 aerosol particles into the atmosphere may not be desirable.
Thus, there is a need for an alternative method of regeneration of SCR catalyst that provides the optimum performance of the catalyst yet minimizes or reduces the amount of SO2 oxidized to SO3 in a power plant. Of particular importance is the composition of the regeneration solution during the neutralization stage of the regeneration process.